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United States Patent |
5,348,662
|
Yen
,   et al.
|
September 20, 1994
|
Process for removing heavy metals from aqueous solutions
Abstract
In the process of removing heavy metals from aqueous solutions
(groundwater) by precipitation of a salt thereof, an oxidizing agent is
optionally used to increase the valence of said metal, and a
precipitation-enhancing agent is added to maximize particle size of the
precipitate and to facilitate its separation from said solution.
Inventors:
|
Yen; Jeffrey H. (Gloucester, NJ);
Spung; Richard C. (Crosby, TX)
|
Assignee:
|
Elf Atochem North America, Inc. (Philadelphia, PA)
|
Appl. No.:
|
061790 |
Filed:
|
May 13, 1993 |
Current U.S. Class: |
210/717; 210/721; 210/724; 210/726; 210/737; 210/911; 210/912; 423/87; 423/92; 423/602; 423/617 |
Intern'l Class: |
C02F 001/62 |
Field of Search: |
210/717,721,724,725,726-728,737,911-914
423/42,43,47,617,87,602,92
|
References Cited
U.S. Patent Documents
3575853 | Apr., 1971 | Gaughan et al. | 210/725.
|
4201667 | May., 1980 | Liao | 210/721.
|
4566975 | Jan., 1986 | Allgulin | 210/711.
|
4622149 | Nov., 1986 | Devuyst et al. | 210/717.
|
4959203 | Sep., 1990 | Knoer et al. | 423/602.
|
5024769 | Jun., 1991 | Gallup | 210/721.
|
5093007 | Mar., 1992 | Domvile | 210/713.
|
5114592 | May., 1992 | Schuster et al. | 210/667.
|
5126116 | Jun., 1992 | Krause et al. | 423/42.
|
5137640 | Aug., 1992 | Poncha | 210/724.
|
5262063 | Nov., 1993 | Yen | 210/724.
|
Foreign Patent Documents |
49-039953 | Apr., 1974 | JP.
| |
Primary Examiner: Hruskoci; Peter A.
Parent Case Text
BACKGROUND OF THE INVENTION
This is a continuation-in-part of application Ser. No. 07/882,777 filed May
14, 1992. This invention concerns a method for the removal of heavy
metals, e.g. arsenic, tin, lead and the like, from aqueous solutions in
which they are dissolved, by the precipitation of the metal in the form of
an insoluble salt. More particularly, it concerns a process for the
removal of heavy metals from contaminated groundwater wherein the heavy
metal is precipitated from solution in the form of its salt, the
precipitate having a maximized particle size to facilitate separation.
Claims
We claim:
1. A process for the removal of at least one heavy metal selected from the
group consisting of arsenic, tin and lead from ground water initially
substantially free of sulfuric acid and contaminated with dissolved said
heavy metal, said process comprising forming a water-soluble salt of said
heavy metal in solution at an acid pH by the addition to said groundwater
of an inorganic water-soluble salt of a metal from Group Ib, IIb or VIII
of the Periodic Table, and then adjusting the pH upward to precipitate
said salt of said heavy metal, the salt formation and precipitation
carried out at a temperature within the range from above 30.degree. C. to
below the boiling point of the reaction solution, optionally, prior to
forming the heavy metal salt, treating said groundwater with an oxidizing
agent in an amount sufficient to convert any low valence said heavy metal
to a higher valency and to oxidize any organic contaminants therein,
before, during or after forming said water-soluble salt of said heavy
metal adding a precipitation-enhancing agent selected from the group
consisting of calcium sulfate, arsenic trioxide, calcium arsenate and
cupric oxide to said groundwater in an amount sufficient to enlarge the
average particle size of the heavy metal salt when precipitated, said
precipitation-enhancing agent being present in the groundwater in
crystalline form during the precipitation of said heavy metal salt, and
separating the precipitate from said groundwater.
2. The process of claim 1 wherein the oxidizing agent is ozone, hydrogen
peroxide, sulfuric acid, nitric acid or hydrochloric acid.
3. The process of claim 2 wherein said oxidizing agent is sulfuric acid.
4. The process of claim 1 wherein said precipitation-enhancing agent is
calcium sulfate or cupric oxide.
5. The process of claim 4 wherein said calcium sulfate is formed in situ by
the addition of sulfuric acid to the groundwater.
6. The process of claim 5 wherein calcium oxide, calcium hydroxide,
calcium-magnesium carbonate or calcium phosphate is dissolved into the
groundwater to provide additional calcium to react with sulfuric acid.
7. The process of claim 1 wherein said heavy metal is arsenic.
8. The process of claim 7 wherein said water-soluble salt is formed by
reacting the arsenic in the groundwater with the inorganic water-soluble
salt at a temperature ranging from about 45.degree. C. to below the
boiling point of said groundwater.
9. The process of claim 8 wherein said inorganic salt of a metal is cupric
nitrate, cupric chloride, copper sulfate or zinc nitrate.
10. The process of claim 9 wherein said precipitation-enhancing agent is
calcium sulfate which is formed in situ by the addition of sulfuric acid
to the groundwater.
11. The process of claim 10 wherein the pH is adjusted upward by the
addition of an alkali metal hydroxide to said groundwater.
12. The process of claim 10 wherein the pH adjusting step and salt
precipitation is carried out at a temperature within the range of about
45.degree. C. to below the boiling point of said groundwater.
13. The process of claim 12 wherein the precipitate is filtered to separate
solids at a temperature within the precipitation temperature range.
14. The process of claim 10 wherein calcium oxide, calcium hydroxide,
calcium-magnesium carbonate or calcium phosphate is dissolved into the
groundwater to provide additional calcium to react with sulfuric acid.
15. The process of claim 8 wherein the pH adjusting step and salt
precipitation is carried out at a temperature within the range of about
45.degree. C. to below the boiling point of said groundwater.
16. The process of claim 11 wherein the precipitate is filtered to separate
solids at a temperature within the precipitation temperature range.
Description
PRIOR ART
It is known to remove heavy metals from water by precipitation of their
salts. For example, U.S. Pat. No. 4,959,203, discloses the precipitation
of copper arsenate from a solution of copper sulfate to which a
water-soluble arsenate solution is added and the resulting solution
neutralized to precipitate copper arsenate. In that disclosure, Example 1
teaches the addition of sulfuric acid to a copper-arsenic-containing
solution to adjust the pH to 2.2. Impurities precipitated by this
procedure were removed prior to raising the pH with caustic to precipitate
the copper arsenate. Other processes wherein cupric nitrate, cupric
chloride, zinc nitrate and other water-soluble metal salts will react with
the heavy metal ions in aqueous solutions may be used to form metal salts
for precipitation at or near neutral pH. These processes form precipitates
which at times are difficult to filter such that undesirably long
filtering times and filter clogging are encountered with the precipitation
processes. As a result, treated effluents may sometimes contain relatively
high heavy metal contaminants.
STATEMENT OF INVENTION
This invention concerns a process for the removal of at least one heavy
metal from groundwater initially substantially free of sulfuric acid and
contaminated with dissolved heavy metal by forming a water-soluble salt of
said heavy metal in solution at an acid pH by the addition of an inorganic
water-soluble salt of a metal from Group Ib, IIb or VIII of the Periodic
Table, and then adjusting the pH upward to precipitate said salt of said
heavy metal, the salt formation and precipitation carried out at a
temperature within the range from above 30.degree. C. to below the boiling
point of the reaction solution, said process comprising, optionally, prior
to forming the heavy metal salt, treating said groundwater with an
oxidizing agent in an amount sufficient to convert any low valence heavy
metal to a higher valency and to oxidize any organic contaminants therein,
before, during or after forming the heavy metal water-soluble salt, adding
a precipitation-enhancing agent to said groundwater in an amount
sufficient to enlarge the average particle size of the heavy metal salt
when precipitated, said precipitation-enhancing agent being present in the
groundwater in crystalline form during the precipitation of said heavy
metal salt.
DETAILED DESCRIPTION OF THE INVENTION
The process of this invention is useful for the removal of heavy metals
from groundwater and to facilitate the filtration of precipitates of the
heavy metal salts produced in the process. It is particularly useful for
the removal of arsenic dissolved in groundwater (or wastewater) which may
also contain organic chemical impurities which impede precipitation of
heavy metals by acting as chelating agents. The contaminated groundwater
of this invention broadly includes industrial wastewater containing heavy
metal(s). These aqueous solutions also contain dissolved inorganic
impurities including, for example, calcium and magnesium. Because of its
exposure to the earth, typical groundwater always contains calcium ions.
The invention is an improvement in the process of removing one or more
heavy metals from groundwater contaminated therewith by forming a
water-soluble salt of a heavy metal in solution at an acidic pH by the
addition to the groundwater of an inorganic water-soluble salt of a metal
from Group Ib, IIb or VIII of the Periodic Table, and then adjusting the
pH of the solution upward to precipitate the heavy metal salt; the
improvement in this process includes an optional step and a critical step
to produce the desired improvement in filterability of the precipitated
product and to enhance the removal efficiency of heavy metals from
groundwater.
Optionally, the groundwater to be treated is oxidized, prior to salt
formation, to convert any heavy metal of lower valency to a higher and
more reactive metal ion. For example, trivalent arsenic, if present, will
be converted to pentavalent arsenic by the oxidizing agent. This
preoxidation will also serve to oxidize any organic impurities in the
aqueous solution which could interfere with the precipitation of the
formed metal salts. Organics which act as chelating agents to bind the
heavy metal and prevent precipitation are especially in need of oxidation.
Oxidizing agents are used which will accomplish the intended purpose and
not themselves interfere with the relevant reaction or precipitation, in
the amounts employed. That is, the oxidizing agents should not raise the
pH of the metal-containing solution to the degree that precipitation will
occur on the addition of the salt-forming agent. Oxidizing agents which
are useful for this process include, for example, ozone, hydrogen
peroxide, sulfuric acid, nitric acid, hydrochloric acid, and the like.
These agents are used in amounts sufficient to convert lower valent heavy
metals to a higher valency and to oxidize organic contaminants
sufficiently to prevent their interference with precipitation by the metal
salt on pH adjustment in the process. In general, amounts of sulfuric acid
or nitric acid as oxidizing agents to be added to the heavy
metal-containing aqueous solution range from 1 g/L to 5 g/L; preferably
1.5 to 2.5 g/L based on the volume of water in the aqueous solution to be
treated.
Of course, if all heavy metal present in the solution is in its higher
valency form and no organics are present in the solution, the oxidizing
step may be omitted.
A precipitation-enhancing agent is critical to the present invention to
provide improved and more rapid filtration of the formed precipitate and
to enhance the removal efficiency of heavy metals. The step of mixing this
agent with the solution maximizes the particle size and can be
accomplished either before, during or after the heavy metal is converted
to the water-soluble salt at acid pH. The precipitation-enhancing agents
include, for example, calcium sulfate, arsenic trioxide, calcium arsenate,
and cupric oxide. Any inorganic salt with a crystal structure similar to
that of the precipitate can also be used as the precipitation enhancing
agent. Sulfuric acid can be used to form calcium sulfate in situ since
typical groundwater contains some calcium. If wastewater to be treated
does not contain calcium or the groundwater contains insufficient calcium
for in situ preparation of a precipitation-enhancing agent in a sufficient
quantity, calcium oxide, calcium hydroxide, calcium-magnesium carbonate or
calcium phosphate may be added to the groundwater for this purpose. It is
preferred that the calcium to arsenic weight ratio be between 0.1 and 1 to
1, more preferably between 0.2 and 0.6 to 1 for the in situ preparation of
calcium sulfate used to enhance precipitation of the heavy metal salt.
Equivalent ratios of calcium to other heavy metals are also used to
enhance their precipitation.
The precipitation-enhancing agents are usually dissolved in the groundwater
when added thereto, or when formed in situ, and precipitated in their
crystalline form either below the pH or at the same pH at which the heavy
metal salt begins to precipitate. Alternatively, the
precipitation-enhancing agent, e.g. cupric oxide, may remain in
crystalline form (undissolved) when added to the groundwater. In any case,
the precipitation-enhancing agent must be present in the groundwater in
crystalline form during the heavy metal salt precipitation.
To facilitate description of this invention, arsenic will be used
hereinafter to represent heavy metals which also include, for example,
lead and tin. These materials will exist in ionic form in the aqueous
solution.
Arsenic is removed from groundwater containing it by reacting the arsenic
in solution with an inorganic water-soluble metal salt wherein the metals
are those of the Groups Ib, IIb, and VIII of the Periodic Table. Preferred
salts are cupric nitrate, cupric chloride, copper sulfate, zinc nitrate,
ferric chloride, ferric nitrate and the like which form water-soluble
metal arsenates in solution at an acid pH, e.g., from about 1 to 2. The
arsenate is precipitated by adjusting the pH of the aqueous solution
upward such that at a pH of about 3 and above, the arsenate will
precipitate as a solid in finely-divided form. The amount of copper or
equivalent salt introduced into the arsenic-containing solution is
determined by the arsenic content of the solution and, based on a
stoichiometric ratio of metal to arsenic, will be from about 1.1 to about
2:1 and preferably from about 1.1:1 to 1.7:1. On addition of the
water-soluble salt to the arsenic solution, the pH of the solution will be
lowered to about 1-2 and is generally maintained between 1 to 3,
preferably about 2 during the reaction stage. The reaction stage is
preferably carried out for 5 minutes to two hours, more preferably about
10-20 minutes, at a temperature ranging from above 30.degree. C.,
preferably about 45.degree. C., to below the boiling point of the reaction
solution.
The precipitation stage, including filtration is preferably and
advantageously carried out within the same temperature range as recited
above for the reaction stage.
The pressure at which the process is operated is not critical, ambient or
atmospheric being preferred.
The reaction is generally carried out with agitation in either a batch or
continuous system, e.g., continuous stirred tank reactor. It is preferable
to have a ditched bottom reactor with an agitator diameter-to-reactor
vessel diameter of 0.4 to 0.55. Agitation during the reaction will be mild
with mostly axial flow and low shear force to avoid shearing of
precipitate particles when precipitation occurs. Examples of the agitators
include, but are not limited to hydrofoil agitators, such as Lightnin A310
and A315, and profiles agitators, such as Mixel Profile propellers TT, TTP
and TTM. It is preferable to locate the injection ports for inorganic
metal salt slightly above the agitator blades. It is also preferred to
have a tubular anchor close to the bottom of the reactor in order to avoid
the accumulation of precipitate at this location.
Adjustment of the pH of the aqueous reaction system upward is accomplished
by adding an alkali or alkaline earth metal hydroxide, ammonia or
equivalent basic material to the mixture. The concentration of the basic
material preferably ranges from about 150 to 500 g/L, preferably about 300
g/L. Addition of the basic material is preferably made slowly to bring the
pH toward or above neutral over an extended period, e.g., 10 to 60
minutes.
Following precipitation of the metal arsenate, the solids of the reactor
slurry are usually separated in a conventional liquid/solid separator,
e.g., a filter press. However, this process is also beneficial for other
forms of separation such as decanting and centrifugation.
EXAMPLES
The following examples are used to demonstrate this invention. Table 1
below lists a typical composition of groundwater treated in the
accompanying examples. The initial pH of the groundwater ranged from 5.5
to 5.7.
TABLE 1
______________________________________
Chemical Concentration (wt.)
______________________________________
Arsenic about 4,000 ppm
Calcium about 1,000 ppm
Chloroform 160 ppb
Chlorobenzene 94 ppb
Alpha BHC* 490 ppb
Beta BHC 95 ppb
Gamma BHC 740 ppb
Delta BHC 410 ppb
______________________________________
* benzene hexachloride
ppm = parts per million
ppb = parts per billion
EXAMPLE 1 (Comparative)
500 g of groundwater (See Table 1) was added to a stirred glass reactor and
the temperature of the reactor slowly raised to about
55.degree.-60.degree. C. (130.degree.-140.degree. F.) at which it was
maintained through the precipitation step. 10.2g of cupric nitrate
[Cu(NO.sub.3).sub.2.2.5H.sub.2 O ] was added to the heated reactor and the
reactor pH dropped to about 2. 11.2 mL of 25 wt. percent sodium hydroxide
were slowly added to the reactor over a period of 20 minutes during which
the pH of the reactor mix reached 7. The stirrer was turned off. The
resulting slurry of small particle size took over one (1) hour to filter
and provided filtrate having 0.76 ppm (wt.) arsenic. Arsenic removed from
the groundwater was calculated to be 99.8%.
EXAMPLE 2
500 g of groundwater (See Table 1) was added to a stirred glass reactor and
the temperature in the reactor slowly raised to about
55.degree.-71.degree. C. (130.degree.-150.degree. F.) at which it was
maintained through the precipitation step. 3.4g of concentrated sulfuric
acid (>95%) was added to the reactor and the pH of the solution in the
reactor dropped to about 1. 10.2g of cupric nitrate was introduced to the
reactor solution and the pH of the resulting mix dropped further to 0.6.
4.9g of sodium hydroxide (NaOH) granulars were slowly added into the
reactor over a period of 23 minutes during which time the pH of the
reactor increased to 4.5. The stirrer motor was turned off and the
precipitate (copper arsenate) showed excellent sedimentation
characteristics. Within 2-3 minutes following termination of agitation,
the sedimentation interface dropped rapidly and the precipitate slurry
occupied only 30% of the volume of the reactant mix, i.e., the reactant
mass consisted of 70% (vol.) clearly supernatant and 30% precipitate
slurry, compared to less than 10% clear supernatant after 2-3 minutes
following agitation termination in Example 1. Filtration was accomplished
in 10-15 minutes providing a filtrate of 0.63 ppm arsenic and over 99.98%
arsenic removal from the groundwater. Compared to Example 1, it is evident
from this example that the addition of sulfuric acid enhances the
filterability of the precipitate and the arsenic removal efficiency.
EXAMPLE 3
3500 g of groundwater (See Table 1) was introduced into a stirred glass
reactor (4500 ml.) and the temperature in the reactor was slowly raised to
about 55.degree.-71.degree. C. (130.degree.-150.degree. F.) at which it
was maintained through the precipitation stage. 13.1g of concentrated
sulfuric acid (>95%) was added to the solution in the reactor dropping the
solution pH to about 1.5. 71.3g of cupric nitrate was then added to the
reactor solution to drop the pH further to 1.3. 31.9g of NaOH granulars
were slowly added to the reactor over 19 minutes raising the pH to 7.5.
After agitation (stirring) was turned off, the precipitate (copper
arsenate) showed excellent sedimentation characteristics. Within a minute
after the agitator was turned off, the sedimentation interface dropped
rapidly and the precipitate slurry occupied only about 65% of the volume
of the reactant mass, i.e., 65% precipitate and 35% clear supernatant
compared to less than 10% clear supernatant in Example 1 observed
following 2-3 minutes after termination of agitation. Within 3 minutes of
agitation termination, the precipitate slurry occupied only 40% of the
volume of the reactant mass and the filtration rate was quite rapid. The
arsenic content of the filtrate was measured at 0.997 ppm.
EXAMPLE 4
Eight hundred grams of the groundwater (with typical chemical contents of
Table 1) was added into a stirred glass reactor. The reactor temperature
was maintained at ambient temperature (about 70.degree. F.). 3.0 grams of
concentrated sulfuric acid (>95%) was added into the reactor and the
reactor pH dropped to about 1.7, followed by the addition of 19.7 grams of
cupric nitrate salt, Cu(NO.sub.3).sub.2.2.5 H.sub.2 O. The reactor pH
further dropped to 1.6. Caustic solution was slowly added into the reactor
and the reactor pH was raised to 6.1. Half of the resulting product was
filtered with a filter paper and was found to be rather difficult to
filter. The filtrate was found to have over 50 ppm arsenic.
The second half of the resulting product was heated up to 120.degree. F.
for fifteen minutes. The slurry was relatively easier to filter using the
same grade filter paper compared to the first half of the product. The
filtrate contained 26.1 ppm arsenic.
EXAMPLE 5
500 g of groundwater (See Table 1) was added to a stirred glass reactor and
the reactor was maintained at ambient temperature (about 80.degree. F.)
through the precipitation step. 3.4g of concentrated sulfuric acid (>95%)
was added to the reactor and the pH of the solution in the reactor dropped
to about 2.3. 19.9g of cupric nitrate containing 14% Cu by weight was
introduced to the reactor solution and the pH of the resulting mix dropped
further to 2.0 20.7g of sodium hydroxide (NaOH) granulars were slowly
added into the reactor over a period of about 15 minutes during which time
the pH of the reactor increased to 7.8. The filtrate had an arsenic
concentration of 45.6 ppm. representing an arsenic removal efficiency of
98.9%. The average particle size of the precipitate was 43.7 .mu.m.
EXAMPLE 6
Sixty gallons of the groundwater (See Table 1) was added into a
steam-jacketed stirred stainless steel reactor. The reactor temperature
was maintained at about 150.degree.-160.degree. F. throughout the
reaction. 230 grams of concentrated sulfuric acid (>95%) was added into
the reactor, followed by the addition of 2,000 grams of cupric nitrate
solution containing 0.26 grams Cu per ml. The reactor pH was dropped to
2.4. Caustic solution was slowly added into the reactor and the reactor pH
was raised to 7.6. The reactant was filtered and the filtrate was found to
have 0.204 ppm arsenic. The average particle size of the precipitate was
66.2 .mu.m.
Compared to Examples 4 and 5, it is evident that the efficiency of arsenic
removal and the average particle size of precipitate can be affected by
reaction and precipitation temperatures. Elevated temperature is preferred
throughout the reaction and precipitation stages.
EXAMPLES 7-11
In Examples 7, 8 and 11, groundwater was treated at elevated temperature in
accordance with the procedure of Example 6 except for the modifications
noted in Table 2 below. Examples 9 and 10 were conducted as follows:
Example 9
One thousand mls. of groundwater containing 2000 ppm arsenic was added to a
stirred glass reactor. The water was heated to 130.degree. F. and stirred
slowly. 12.6 mls aqueous copper nitrate solution was added to the solution
(a copper: arsenic ratio of 1.8:1). The pH dropped from 6.1 to 1.5. One
quarter gram of black cupric oxide powder was added to the reactor. There
was no change in pH or temperature. Twenty five percent caustic soda
solution was dripped into the reactor, at the rate of 20 mls/hour. After
30 minutes, 10.6 mls were added and the pH was 7.1.
EXAMPLE 10
The experiment reported in Example 9 was repeated up to the introduction of
the caustic soda solution. Then, twenty-five percent caustic soda solution
was dripped into the beaker at the rate of 40 mls/hour. After 14 minutes,
10.2 mls had been added and the pH was 7.1.
The average particle size obtained in the experiments of Examples 7-11 are
reported in Table 2 below.
TABLE 2
______________________________________
Example No.
Process Conditions
7 8 9 10 11
______________________________________
As* in Groundwater
4000 2000 2000 2000 2000
(ppm)
Precipitation Enhanc-
None None CuO** CuO** H.sub.2 SO.sub.4
ing Agent
Neutralizing Agent
NaOH NaCO.sub.3
NaOH NaOH NaOH
Avg. Particle Size
37.3 19.9 54.5 61.3 66.2
of Precipitate (.mu.m)
______________________________________
*Arsenic
** Black Cupric Oxide Powder
It is evident from the above results that the average particle size of the
precipitate is enlarged by the use of sulfuric acid (to form calcium
sulfate in situ) or cupric oxide.
Furthermore, elevated reaction and precipitation temperatures can improve
the arsenic removal efficiency and the average particle size of the
precipitate.
EXAMPLE 12
100 gallons of groundwater as typified in Table 1 was added to a
steam-jacketed stirred stainless steel reactor. The groundwater contained
1,910 ppm of arsenic, by weight. The reactor temperature was maintained at
about 140.degree.-150.degree. F. throughout the reaction and mild stirring
was continuous. 383.5 grams of concentrated sulfuric acid (>95%) was added
to the reactor followed by the addition of 4.3L of cupric nitrate solution
(0.263g. Cu/mL) in four steps. A small sample taken from the reactor was
neutralized with caustic and the resulting precipitate was milky and
difficult to filter. An additional 296 g. of concentrated sulfuric acid
(>95%) was added into the reactor followed by 471 g. of lime (CaO) and 220
g. of concentrated sulfuric acid. A small sample was then taken from the
reactor and neutralized with caustic. The precipitate settled easily and
showed a better particle size distribution compared to the first sample.
The reactant in the reactor was neutralized with caustic to a pH of 7. The
precipitate was easily filtered through a filter press clearly
demonstrating the importance of the in situ prepared
precipitation-enhancing agent in the reactant.
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